U.S. patent number 4,692,726 [Application Number 06/890,686] was granted by the patent office on 1987-09-08 for multiple resonator dielectric filter.
This patent grant is currently assigned to Motorola, Inc.. Invention is credited to David M. De Muro, Steven R. Green, Raymond L. Sokola.
United States Patent |
4,692,726 |
Green , et al. |
September 8, 1987 |
Multiple resonator dielectric filter
Abstract
A multiresonator dielectric block filter is disclosed in which
capacitive coupling between foreshortened resonators disposed in
the dielectric block is controlled by an electrode strip coupled to
the conductive material covering the majority of the dielectric
block surface. The electrode strip extends at least partially
between two adjacent resonators to control the capacitive coupling
between the resonators.
Inventors: |
Green; Steven R. (Glendale
Hts., IL), De Muro; David M. (Schaumburg, IL), Sokola;
Raymond L. (Lake Zurich, IL) |
Assignee: |
Motorola, Inc. (Schaumburg,
IL)
|
Family
ID: |
25397003 |
Appl.
No.: |
06/890,686 |
Filed: |
July 25, 1986 |
Current U.S.
Class: |
333/206; 333/202;
333/207; 333/223 |
Current CPC
Class: |
H01P
1/2136 (20130101); H01P 1/2056 (20130101) |
Current International
Class: |
H01P
1/213 (20060101); H01P 1/205 (20060101); H01P
1/20 (20060101); H01P 001/202 (); H01P
007/04 () |
Field of
Search: |
;333/202,204,205,206,208-212,219,222-224,227,235,245,246,248,207,203 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Nussbaum; Marvin L.
Attorney, Agent or Firm: Jenski; Raymond A. Hackbart;
Rolland R. Southard; Donald B.
Claims
We claim:
1. A filter comprising:
dielectric means comprised of a dielectric material and having
first, second, and side surfaces, said second and side surfaces of
the dielectric means being substantially covered with a conductive
material;
at least first, second, and third holes having surfaces
substantially covered by a conductive material, extending from the
first surface of the dielectric means toward the second surface
thereof and having openings on the first surface of the dielectric
means that are disposed at predetermined distances relative to one
another and substantially aligned with one another;
first and second coupling means coupled to said first and third
holes, respectively;
first, second, and third capacitive means each including electrode
means coupled to and surrounding the openings of first, second, and
third holes, respectively, for capacitively coupling said first
hole to said second hole and said second hole to said third hole
and capacitively coupling said holes to the conductive material on
the side surfaces of said dielectric means; and
strip electrode means coupled to the conductive coating on the side
surfaces of said dielectric means and extending at least partially
between two of said holes for adjusting the capacitive coupling
therebetween.
2. The filter according to claim 1, wherein said first and second
coupling means include first and second electrode means disposed on
the top surface of the dielectric means near, and coupled to said
first and third holes, respectively.
3. The filter according to claim 2, wherein said first and second
electrode means each include a portion of the top surface of the
dielectric means covered with said conductive material.
4. The filter according to claim 1, further including fourth
capacitive means having electrode means interposed between and
capacitively intercoupling said electrode means of the first and
second capacitive means.
5. The filter according to claim 1, further including a fourth hole
having surfaces substantially covered by a conductive material,
extending from the first surface of the dielectric means toward the
second surface thereof and having an opening on the first surface
of the dielectric means disposed at a predetermined distance
relative to said third hole and substantially aligned with
same.
6. The filter according to claim 5 further including fourth
capacitive means surrounding the opening of the fourth hole for
capacitively coupling said fourth hole to the conductive material
on the side surfaces of said dielectric means and said second
coupling means.
7. A filter comprising:
dielectric means comprised of a dielectric material and having
first, second, and side surfaces, said second and side surfaces of
the dielectric means being substantially covered with a conductive
material;
at least first, second, and third holes having surfaces
substantially covered by a conductive material, extending from the
first surface of the dielectric means toward the second surface
thereof and having openings on the first surface of the dielectric
means that are disposed at predetermined distances relative to one
another and substantially aligned with one another;
first and second coupling means coupled to said first and third
holes, respectively;
first and second capacitive means each including electrode means
coupled to and surrounding the openings of first and second holes,
respectively, for capacitively coupling said first hole to said
second hole and capacitively coupling said holes to the conductive
material on the side surfaces of said dielectric means;
third capacitive means including electrode means coupled to and
surrounding the opening of the third hole for capacitively coupling
said third hole to the conductive material on the side surfaces of
said dielectric means;
first strip electrode means coupled to the conductive material on
the side surfaces of said dielectric means and extending between
said second and third holes substantially preventing capacitive
coupling therebetween; and
second strip electrode means coupled to the conductive material on
the side surfaces of said dielectric means and extending at least
partially between said first and second holes for adjusting the
capacitive coupling therebetween.
8. The filter according to claim 7, wherein said first and second
coupling means include first and second electrode means disposed on
the top surface of the dielectric means near, and coupled to said
first and third holes, respectively.
9. The filter according to claim 8, wherein said first and second
electrode means each include a portion of the top surface of the
dielectric means covered with said conductive material.
10. The filter according to claim 7, further including fourth
capacitive means having electrode means interposed between and
capacitively intercoupling said electrode means of the first and
second capacitive means.
11. The filter according to claim 7, further including a fourth
hole having surfaces substantially covered by a conductive
material, extending from the first surface of the dielectric means
toward the second surface thereof and having an opening on the
first surface of the dielectric means disposed at a predetermined
distance relative to said third hole and substantially aligned with
same.
12. The filter according to claim 11 further including fourth
capacitive means surrounding the opening of the fourth hole for
capacitively coupling said fourth hole to the conductive material
on the side surfaces of said dielectric means and said second
coupling means.
13. A multi-passband filter for coupling radio-frequency (RF)
signals between an antenna and first and second RF signal
utilization means, comprising:
(a) a first filter comprising:
dielectric means comprised of a dielectric material and having
first, second, and side surfaces, said second and side surfaces of
the dielectric means being substantially covered with a conductive
material;
at least first, second, and third holes having surfaces
substantially covered by a conductive material, extending from the
first surface of the dielectric means toward the second surface
thereof and having openings on the first surface of the dielectric
means that are disposed at predetermined distances relative to one
another and substantially alligned with one another;
first and second coupling means coupled to said first and third
holes, respectively, said first coupling means further being
coupled to a first RF signal utilization means;
first, second, and third capacitive means each including electrode
means coupled to and surrounding the openings of first, second, and
third holes, respectively, for capacitively coupling said first
hole to said second hole and said second hole to said third hole
and capacitively coupling said holes to the conductive material on
the side surfaces of said dielectric means; and
strip electrode means coupled to the conductive material on the
side surfaces of said dielectric means and extending at least
partially between two of said holes for adjusting the capacitive
coupling therebetween;
(b) a second filter comprising:
dielectric means comprised of a dielectric material and having
first, second, and side surfaces, said second and side surfaces of
the dielectric means being substantially covered with a conductive
material;
at least first, second, and third holes having surfaces
substantially covered by a conductive material, extending from the
first surface of the dielectric means toward the second surface
thereof and having openings on the first surface of the dielectric
means that are disposed at predetermined distances relative to one
another and substantially aligned with one another;
first and second coupling means coupled to said first and third
holes, respectively, said first coupling means further being
coupled to a second RF signal utilization means;
first, second, and third capacitive means each including electrode
means coupled to and surrounding the openings of first, second and
third holes, respectively, for capacitively coupling said first
hole to said second hole and said second hole to said third hole
and capacitively coupling said holes to the conductive material on
the side surfaces of said dielectric means; and
(c) first transmission line means coupled between the second
coupling means of said first filter and the antenna; and
(d) second transmission line means coupled between the second
coupling means of said second filter and the antenna.
14. A multi-passband filter for coupling a radio-frequency (RF)
signal from a RF transmitter to an antenna and coupling another RF
signal from the antenna to an RF receiver, comprising:
(a) a first filter comprising:
dielectric means comprised of a dielectric material and having
first, second, and side surfaces, said second and side surfaces of
the dielectric means being substantially covered with a conductive
material;
at least first, second, and third holes having surfaces
substantially covered by a conductive material, extending from the
first surface of the dielectric means toward the second surface
thereof and having openings on the first surface of the dielectric
means that are disposed at predetermined distances relative to one
another and substantially aligned with one another;
first and second coupling means coupled to said first and third
holes, respectively, said first coupling means further being
coupled to said RF transmitter;
first and second capacitive means each including electrode means
coupled to and surrounding the openings of first and second holes,
respectively, for capacitively coupling said first hole to said
second hole and capacitively coupling said holes to the conductive
material on the side surfaces of said dielectric means;
third capacitive means including electrode means coupled to and
surrounding the opening of the third hole for capacitively coupling
said third hole to the conductive material on the side surfaces of
said dielectric means;
first strip electrode means coupled to the conductive material on
the side surfaces of said dielectric means and extending between
said second and third holes substantially preventing capacitive
coupling therebetween; and
second strip electrode means coupled to the conductive material on
the side surfaces of said dielectric means and extending at least
partially between said first and second holes for adjusting the
capacitive coupling therebetween;
(b) a second filter comprising:
dielectric means comprised of a dielectric material and having
first, second, and side surfaces, said second and side surfaces of
the dielectric means being substantially covered with a conductive
material;
at least first, second, and third holes having surfaces
substantially covered by a conductive material, extending from the
first surface of the dielectric means toward the second surface
thereof and having openings on the first surface of the dielectric
means that are disposed at predetermined distances relative to one
another and substantially aligned with one another;
first and second coupling means coupled to said first and third
holes, respectively, said first coupling means further being
coupled to said RF receiver;
first and second capacitive means each including electrode means
coupled to and surrounding the openings of first and second holes,
respectively, for capacitively coupling said first hole to said
second hole and capacitively coupling said holes to the conductive
material on the side surfaces of said dielectric means;
third capacitive means including electrode means coupled to and
surrounding the opening of the third hole for capacitively coupling
said third hole to the conductive material on the side surfaces of
said dielectric means;
(c) first transmission line means coupled between the second
coupling means of said first filter and the antenna; and
(d) second transmission line means coupled between the second
coupling means of said second filter and the antenna.
Description
BACKGROUND OF THE INVENTION
The present invention is related generally to radio frequency (RF)
filters, and more particularly to a dielectric block band pass
filter having improved capacitive inter-resonator coupling via
metalization which produces a filter that is particularly well
adapted for use in mobile and portable radio transmitting and
receiving devices. This invention is related to the invention
disclosed in U.S. patent application Ser. No. 890,682 filed on the
same data as the present invention.
Conventional dielectric filters offer advantages in physical and
electrical performance which make them ideally suited for use in
mobile and portable radio transceivers. Conventional
multi-resonator filters include a plurality of resonators that are
typically forshortened short-circuited quarter-wavelength coaxial
or helical transmission lines. The resonators are arranged in a
conductive enclosure and may be inductively coupled one to another
by apertures in their common walls. Other conventional filters may
employ purely inductive coupling between resonators in a common
dielectric block of material by preventing capacitive coupling
between resonators. Capacitive coupling between dielectric block
filter resonators has been employed in some types of filters (see
U.S. patent application Ser. No. 656,121, "Single-Block
Dual-Passband Ceramic Filter", filed in behalf of Kommrusch on
Sept. 27, 1984), now abandoned. However, when a precise filter
response is required, it has been found to be expensive to maintain
the desired capacitive coupling for the filter response.
Additionally, when a modification of filter parameters is needed, a
complete redesign of the filter physical characteristics has
traditionally been necessary.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a
dielectric filter having an improved capacitive coupling.
It is another object of the present invention to enable a
dielectric filter to have its filter characteristics modified by
changing metalization coupling the resonators.
It is a further object of the present invention to couple improved
dielectric filters in a configuration which enables their
performance as a radio transceiver duplexer.
Therefore, as briefly described, the present invention encompasses
a substrate mountable filter comprising a dielectric filter having
its surfaces substantially covered with a conductive material
except for a first surface. A plurality of holes extend from the
first surface to a second surface and are substantially covered by
a conductive material which extends from the first surface toward
the second surface. Electrodes surround the openings of the holes
on the first surface and capacitively couple the holes sequentially
to each other and to the conductive material covering the other
surfaces of the dielectric filter. A strip electrode, coupled to
the conductive material, extends at least partially between two of
said holes for adjusting the capacitive coupling between the
holes.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a conventional dielectric filter
illustrating the orientation of the resonator elements and the
input/output coupling.
FIGS. 2, 3, and 4 are sectional views of FIG. 1 illustrating
metalization patterns which may be employed in the resonator
holes.
FIG. 5 is a bottom perspective of a dielectric block filter and
mounting bracket employing the present invention.
FIG. 6 is a sectional view illustrating an input or output terminal
employed in the present invention.
FIG. 7 is a dimensional diagram of the mounting bracket employed in
the present invention.
FIG. 8 is a dimensional view of a printed circuit board mounted
duplexer employing component-mountable filters.
FIG. 9 is a schematic diagram of a component-mountable filter.
FIG. 10 is a schematic diagram of the duplexer of FIG. 8.
FIG. 11 is a schematic diagram of a printed circuit mounted
duplexer employing component-mountable filters in a diversity
receive antenna configuration.
FIGS. 12A, 12B, 12C, 12D, and 12E illustrate metalization patterns
which may be employed in the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
In FIG. 1, there is illustrated a dielectrically loaded band pass
filter 100 employing a conventional input connector 101 and a
conventional output connector 103. Such a filter is more fully
described in U.S. Pat. No. 4,431,977 "Ceramic Band Pass Filter" and
assigned to the assignae of the present invention and incorporated
by reference herein. Filter 100 includes a block 105 which is
comprised of a dielectric material that is selectively plated with
a conductive material. Filter 100 is generally constructed of a
suitable dielectric material such as a ceramic material which has
low loss, a high dielectric constant, and a low temperature
coefficient of the dielectric constant. In the preferred
embodiment, filter 100 is comprised of a ceramic compound including
barium oxide, titanium oxide and ziconium oxide, the electrical
characteristics of which are similar to those described in more
detail in an article by G. H. Jonker and W. Kwestroo, entitled "The
Ternery Systems BaO-TiO.sub.2 -ZrO.sub.2 ", Published in the
Journal of the American Ceramic Society, Volume 41, no. 10 at pages
390-394, October, 1958. Of the ceramic compounds described in this
article, the compound in table VI having the composition 18.5 mole
percent BaO, 77.0 mole percent TiO.sub.2 and 4.5 mole percent
ZrO.sub.2 and having a dielectric constant of approximately 40 is
well suited for use in the ceramic of the present invention.
A dielectric filter such as that of block 105 of Filter 100 is
generally covered or plated, with the exception of areas 107, with
an electrically conductive material such as copper or silver. A
filter such as block 105 includes a multitude of holes 109 which
each extend from the top surface to the bottom surface thereof and
are likewise plated with an electrically conductive material. The
plating of the holes 109 is electrically common with the conductive
plating covering the block 105 at one end of the holes 109 and
isolated from the plating covering the block 105 at the opposite
end of the holes 109. Further, the plating of holes 109 at the
isolated end may extend onto the top surface of block 105. Thus,
each of the plated holes 109 is essentially a foreshortened coaxial
resonator comprised of a short coaxial transmission line having a
length selected for desired filter response characteristics.
(Although the block 105 is shown in FIG. 1 with six plated holes,
any number of plated holes may be utilized depending upon the
filter response characteristics desired).
The plating of holes 109 in the filter block 105 is illustrated
more clearly by the cross-section through any hole 109. Conductive
plating 204 on dielectric material 202 extends through hole 201 to
the top surface with the exception of a circular portion 240 around
hole 201. Other conductive plating arrangements may also be
utilized, two of which are illustrated in FIGS. 3 and 4. In FIG. 3,
conductive plating 304 on dielectric material 302 extends through
hole 301 to the bottom surface with the exception of portion 340.
The plating arrangement in FIG. 3 is substantially identical to
that in FIG. 2, the difference being that unplated portion 340 is
on the bottom surface instead of on the top surface, in FIG. 4,
conductive plating 404 on dielectric material 402 extends partially
through hole 401 leaving part of hole 401 unplated. The plating
arrangement in FIG. 4 can also be reversed as in FIG. 3 so that the
unplated portion 440 is on the bottom surface.
Coupling between the plated hole resonators is accomplished through
the dielectric material and may be varied by varying the width of
the dielectric material and the distance between adjacent coaxial
resonators. The width of the dielectric material between adjacent
holes 109 can be adjusted in any suitable regular or irregular
manner, such as, for example, by the use of slots, cylindrical
holes, square or rectangular holes, or irregularly shaped
holes.
As shown in FIG. 1, RF signals are capacitively coupled to and from
the dielectric filter 100 by means of input and output electrodes
111 and 113, respectively, which, in turn, are coupled to input and
output connectors 101 and 103, respectively.
The resonant frequency of the coaxial resonators provided by plated
holes 109 is determined primarily by the depth of the hole,
thickness of the dielectric block in the direction of the hole, and
the amount of plating removed from the top of the filter near the
hole. Tuning of filter 100 may be accomplished by the removal of
additional ground plating or resonator plating extending upon the
top surface of the block 105 near the top of each plated hole. The
removal of plating for tuning the filter can easily be automated,
and can be accomplished by means of a laser, sandblast trimmer, or
other suitable trimming devices while monitoring the return loss
angle of the filter.
Referring now to FIG. 5, a dielectric filter employing the present
invention is shown in a exploded perspective view. A block of
dielectric material 501 is placed in a carrying bracket 503 which
performs the multiple functions of providing a rigid mounting
platform such that dielectric block 501 may be inserted into a
printed circuit board or other substrate, providing simplified
input and output connections via feed through terminals 505 and
507, and providing positive ground contact between the conductive
outer surface of dielectric block 501 and bracket 503 via contacts
509, 510, 511, 512, and other contacts not shown. Contacts 509 and
510 additionally provide a dielectric block 501 locating function
within the bracket 503. Mounting bracket 503 further provides
mounting tabs 515-525 to locate and support the bracket and filter
on a mounting substrate and provide positive ground contact for
radio frequency signals from the mounting bracket 503 to the
receiving mounting substrate. A mounting bracket for a dielectric
filter has been disclosed in U.S. patent application Ser. No.
656,121, "Single-Block Dual-Passband Ceramic Filter", filed in
behalf of Kommrusch on Sept. 27, 1984 and assigned to the assignee
of the present invention. This previously disclosed bracket,
however, does not provide the simplified mounting of the bracket of
the present invention.
In one preferred embodiment the dielectric filter 501 consists of a
ceramic material and utilizes seven internally plated holes as
foreshortened resonators to produce a band pass filter for
operation in radio bands reserved for cellular mobile telephone. In
this embodiment the conductive plating covering the ceramid block
501 extends conformally on all surfaces except that on which the
resonator plating is wrapped from the holes onto the outer surface.
Thus, holes 529-535 have corresponding plating 537-543 metallized
on the outer surface of block 501. These areas 537-543 are
electrically separate from the ground plating but provide
capacitive coupling to the ground plating. Additionally, an input
plated area 547 and an output plated area 549 provide capacitive
coupling between the input terminal 505 and the coaxial resonator
formed from the internally plated hole 529 and its externally
plated area 537 while plated area 549 provides capacitive coupling
between the output terminal 507 and the output resonator formed
from plated hole 535 and external plated area 543. Ground stripes
553-558 are plated between the coaxial resonator plated holes in
order that inter-resonator coupling is adjusted.
Ceramic block 501 is inserted into bracket 503 with the externally
plated resonator areas 537-543 oriented downward into the bracket
503 such that additional shielding is afforded by the bracket 503.
Input mounting pin 505 is connected to plated area 547 and output
terminal 507 is connected to plated area 549 as shown in FIG. 6.
Input terminal 505, which may be a low shunt capacity feed through
such as a 100B0047 terminal manufactured by Airpax Electronics
Inc., consists of a solderable eyelet 601 and insulating glass bead
603 supporting a center conductor 605. The eyelet 601 is
conductively bonded to bracket 503 to provide a secure mounting for
the input connector 505. The center conductor 605 is brought into
contact with plated area 547 by the dimensions of the bracket 503
and the block 501. The center conductor 605 is soldered or
otherwise conductively bonded at one end to area 547 to provide a
reliable RF connection to plated area 547. The other end of the
center conductor 605 may then be easily soldered or plugged into a
substrate which holds the mounting bracket 503. A similar
construction is employed for output terminal 507 and its associated
plated area 549.
A detail of the mounting bracket 503 is shown in FIG. 7. The
spacing of the mounting tabs 515-525 is shown in detail for the
preferred embodiment. These spacings are important at the
frequencies of operation of this filter in order to maintain
maximum ultimate attenuation. Low ground path inductance in the
mounting bracket is realized by placing mounting tabs 517 and 519
close to the input and output ports (505 and 507 of FIG. 5
respectively) and the remainder of the tabs above the side and
bottom of the bracket 503. Connection between the dielectric block
501 and bracket 503 is assured near the input and output terminals
by contacts similar to contacts 511 and 512 located close to the
terminals. All contacts, 509, 510, 511, and 512 (and the equivalent
contacts on the opposite side of the brackets not shown) may be
soldered or otherwise bonded to the dielectric block 501 such that
electrical connection may be permanently assured.
It can be readily ascertained that the position of the tabs 518,
520, and 521 are asymmetrical. Also, the input/output terminals 505
and 507 are offset from the centerline of the bracket 503. This
asymmetry enables a "keying" of the bracket 503 so that a filter
can be inserted in a printed circuit board or other substrate in
only one orientation.
One unique aspect of the present invention is shown in FIG. 8. A
dielectric filter block such as block 501 is mounted in bracket 503
and becomes a unitized circuit component which may be inserted into
a printed circuit board or substrate 801. Appropriate holes 803 and
805 are located on the printed circuit board 801 to accept the
input and output terminals 505 and 507 (not shown in FIG. 8),
respectively. Further, appropriately located slots 815-825 are
located in the printed circuit board 801 to accept the
corresponding tabs of the bracket 503. Thus the filter 501 and
bracket 503 may be mounted on a circuit board 801 like any other
component and circuit runners may extend from the input hole 803
and the output hole 805 such that the filter may be electrically
connected to other circuitry with a minimum of effort. The circuit
board runners, 807 and 809, may be constructed as stripline or
microstrip transmission lines to yield improved duplexer
performance.
Referring to FIG. 9, there is illustrated an equivalent circuit
diagram for the dielectric filter 501 utilized as a band pass
filter. An input signal from a signal source may be applied via
terminal 505 to input electrode 547 in FIG. 5, which corresponds to
the common junction of capacitors 924 and 944 in FIG. 9. Capacitor
944 is the capacitance between electrode 547 and the surrounding
ground plating, and capacitor 924 is the capacitance between
electrode 547 and the coaxial resonator provided by plated hole 529
in FIG. 5. The coaxial resonators provided by plated 529-535 in
FIG. 5 correspond to shorted transmission lines 929-935 in FIG. 9.
Capacitors 937-943 in FIG. 9 represent the capacitance between the
coaxial resonators provided by the extended plating 537-543 of the
plated holes in FIG. 5 and the surrounding ground plating on the
top surface. Capacitor 925 represents the capacitance between the
resonator provided by plated hole 535 and electrode 549 in FIG. 5,
and capacitor 945 represents the capacitance between electrode 549
and the surrounding ground plating. An output signal is provided at
the junction of capacitors 925 and 945, and coupled to output
terminal 547 for utilization by external circuitry.
Referring now to FIG. 10, there is illustrated a multi-band filter
comprised of two intercoupled dielectric band pass filters 1004 and
1012 and employing the present invention. Two or more of the
inventive band pass filters may be intercoupled on a printed
circuit board of substrate to provide apparatus that combines
and/or frequency sorts two RF signals into and/or from a composite
RF signal. In one application of the preferred embodiment the
present invention is employed in the arrangement of FIG. 10 which
couples a transmit signal from an RF transmitter 1002 to an antenna
1008 and a receive signal from antenna 1008 to an RF receiver 1014.
The arrangement in FIG. 10 can be advantageously utilized in
mobile, portable, and fixed station radios as an antenna duplexer.
The transmit signal from RF transmitter 1002 is coupled to filter
1004 by a transmission line 1005, realized by the plated runner 807
of FIG. 8 on the printed circuit board in the preferred embodiment,
and the filtered transmit signal is coupled via circuit board
runner transmission line 1006 (runner 809 of FIG. 8) to antenna
1008. Filter 1004 is a ceramic band pass filter of the present
invention, such as the filter illustrated in FIGS. 5 and 8. The
pass band of filter 1004 is centered about the frequency of the
transmit signal from RF transmitter 1002, while at the same time
greatly attenuating the frequency of the received signal. In
addition, the length of transmission line 1006 is selected to
maximize its impedance at the frequency of the received signal.
A received signal from antenna 1008 in FIG. 10 is coupled by
transmission line 1010, also realized as a printed circuit board
runner, to filter 1012 and thence via circuit board runner
transmission line 1013 to RF receiver 1014. Filter 1012, which also
may be one of the inventive band pass filters illustrated in FIGS.
5 and 8, has a pass band centered about the frequency of the
receive signal, while at the same time greatly attenuating the
transmit signal. Similarly, the length of transmission line 1010 is
selected to maximize its impedance at the transmit signal frequency
for further attenuating the transmit signal.
In the embodiment of the RF signal duplexing apparatus of FIG. 10,
transmit signals having a frequency range from 825 MHz to 851 MHz
and receive signals having a frequency range from 870 MHz to 896
MHz are coupled to the antenna of a mobile radio. The dielectric
band pass filters 1004 and 1012 utilize a dielectric of ceramic and
are constructed in accordance with the present invention as shown
in FIG. 5. The filters 1004 and 1012 each have a length of 3.0 inch
and a width of 0.45 inch. The height is a primary determinant of
the frequency of operation and, in the preferred embodiment, is
0.49 inch in the transmit filter 1004 and 0.44 inch in the receive
filter 1012. Filter 1004 has an insertion loss of 2.5 dB and
attenuate receive signals by at least 50 dB. Filter 1012 has an
insertion loss of 3.0 dB and attenuates receive signals by at least
60 dB. An alternative interconnection of the circuit board
monostable dielectric block filters is shown in FIG. 11.
It is sometimes desirable to utilize two switchable antennas for a
receiver so that the antenna receiving the best signal may be
switchably coupled to the receiver and provide the well-known
antenna diversity function. By not providing a transmission line
coupling directly between transmission lines 1006 and 1010 (at
point A) but by inserting an antenna switch 1101 selecting a shared
transmit/receive antenna 1103 and a receive only antenna 1105
between the antennas, the separate transmit and receive filters
1004 and 1012 may be coupled by 180.degree. reflection coefficient
transmission lines 1107 and 1109 in a fashion to provide a
diversity receive function.
The filter operational characteristics may be determined by the
metallization pattern employed on the surface of the dielectric
block which is not fully metallized. Dielectric filters such as
described herein are instrinsically coupled by inductance. That is,
the magnetic fields in the dielectric material govern the coupling.
The inductance may be changed, and even overcome, by introducing
capacitive between the resonators. Referring again to FIG. 5, it
can be seen that a seven pole configuration is realized by serially
coupling the resonators created by the metallized holes 529-535 and
surface plating 539-543. As shown, the capacitive coupling between
the resonators is restricted by the grounded strip electrodes
554-557. Capacitive coupling by metalization gaps or additional
metalization islands has been shown in the aformentioned U.S.
patent application Ser. No. 656,121 by Kommrusch filed Sept 27,
1984. According to one novel aspect of the present invention, a
controlled capacitive coupling may be achieved by providing
incomplete strip electrodes running on the surface of the
dielectric block between two resonators. In the preferred
embodiment, incomplete strip electrodes 553 and 558, between input
resonator and output resonator and the other resonators, provide a
controlled capacitive coupling to enable combined inductive and
capacitive coupling between adjacent resonators. In practice, the
use of inductive or capacitive coupling provides steeper filter
attenuation skirts on either the high side of the filter passband
or the low side of the filter passband, respectively.
When the dielectric filter blocks are combined as a duplexer filter
as shown diagrammatically in FIG. 10, it is advantageous to employ
a filter having a step attenuation skirt above the passband as the
filter passing the lower frequencies. Also it is advantageous to
employ a filter having a steep attenuation skirt below the passband
as the filter passing the higher frequencies. In this way,
additional protection of transmit and receive paths from each other
can be realized without additional filter resonator elements.
An advantage of the dielectric filter blocks of the present
invention is that the number and spacing of resonators used in the
transmitter filter 1004 (of FIG. 10) may be equal to the number and
spacing of the resonators in the receive filter 1012. The type of
coupling is determined by the metalization pattern employed. The
transmit filter 1004 utilizes inductive coupling between resonators
as illustrated in the metalization pattern of FIG. 12A. The
capacitive coupling between the middle resonators is reduced by the
complete strip electrodes while the input and output resonators
utilize more capacitance in the incomplete strip electrodes in
their coupling to the middle resonators. The receive filter 1012
utilizes capacitive coupling between resonators as illustrated in
the metalization pattern of FIG. 12B. Capacitive coupling is
enabled by the unblocked metalized resonators. (Capacitive coupling
may be enhanced by metalization island such as shown in FIG.
12C).
A novel feature of the present invention creates the ability of the
coupling to be changed by changing the metalization. Additionally,
the mode of resonator operation may be changed from band pass to
band stop by utilizing one or more resonators as a transmission
zero rather than as a transmission pole. Transmission zero
realization by metalization change only is shown in FIG. 12D. The
output electrode 1203 is coupled to the first transmission pole
resonator 1205 by metalization runner 1207. Coupling is also
realized from output electrode 1203 to transmission zero resonator
1209. In the embodiment shown, the transmission zero is tuned to
the low side of the passband to realize additional rejection on the
low side of the passband. A filter utilizing metalization such as
that shown in FIG. 12D would be suitable for use in a duplexer such
as described above.
Additional zeros may be created by proper coupling to other
resonators. Such coupling is shown in the metalization of FIG.
12E.
In summary, then, a multiple resonator dielectric filter has been
shown and described. This filter utilizes metalized hole resonators
having coupling characteristics determined by the metalization
pattern on one surface of the dielectric block. The dielectric
block is metalized with a conductive material on all but one
surface from which the hole resonators extend into the dielectric
block. Electrode metalization around the holes provides capacitive
coupling to this conductive material and from one resonator to an
adjacent resonator. Capacitive coupling between the resonators is
controlled by an electrode at least partially between two adjacent
hole resonators to adjust the capacitive coupling between the
resonators. Therefore, while a particular embodiment of the
invention has been described and shown, it should be understood
that the invention is not limited thereto since many modifications
may be made by those skilled in the art. It is therefore
contemplated to cover any and all such modifications that fall
within the true spirit, and scope of the basic underlying
principles disclose and claimed herein.
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